The mechanisms involved in B-cell activation and isotype switching in the germinal centers of Peyer's patches are poorly understood. In an attempt to better define some of the factors involved, studies in mice have been underway to characterize the B-cell-associated T-cells in mouse Peyer's patches by flow cytometry and to isolate B-cell associated stromal cells. In our laboratory, we have used frozen section immunohistochemistry to further characterize the various T-cell subsets found by flow cytometry. The role of other factors such as transforming growth factor-beta1 is currently being assessed.